Membranes for the separation of gas mixtures by selective permeability are well known and have been in commercial use for a considerable period of time. These membranes are commonly sold as modules designed to present a large membrane surface area to the flowing gas. The modules are used on-site by plants or operations where a gas stream rich in a particular gaseous component is needed and where the available on-site source is a mixture of gases containing the desired component in insufficient concentration. Examples of the types of separations that can be performed by these membranes are nitrogen and oxygen from air; helium from a mixture of helium and nitrogen; hydrogen and carbon dioxide from a hydrogen/carbon dioxide mixture; hydrogen and methane from natural gas; carbon dioxide and methane from natural gas and similar mixtures; and argon from mixtures of argon and nitrogen.
The membranes have been used in a variety of configurations and dimensions, the most prominent of which are hollow fibers and sheets. Hollow fiber membranes are comparable in diameter to the human hair. Modules of these fibers contain several hundred thousand or more such fibers arranged in bundles for parallel flow, the modules containing passages to direct the flow of incoming gas through the membranes and to collect the permeate and remainder streams from separate outlets. In modules designed for boreside feed, the incoming gas is directed to the fiber lumens, whereas in modules designed for shellside feed, the incoming gas is directed to the fiber exteriors, the permeate being drawn from the shellside or boreside, respectively. In the typical module of a sheet-form membrane, the membrane is wound in a spiral wrap around a central pipe, the spiral further containing one or more spacer sheets interleaved with the membrane sheet. The incoming gas is fed to the spiral either from its outside surface to permeate inward or from the central pipe to permeate outward, with the permeate in either case flowing radially through the spiral and either the permeate, the remainder, or both being collected accordingly.
Forcing the incoming gas across these membranes requires a pressure differential which is typically from about 2 (13.8 kPa) to about 10 pounds per square inch (69 kPa). The inlet pressure will be selected to meet this differential while achieving the desired delivery pressures (the pressures of the permeate and the remainder leaving the module), which will generally range from about 25 psia (75 kPag, 175 kPa) to about 190 psia (1210 kpag, 1310 kPa). Inlet pressures will most often range from about 60 psia (314 kPag, 414 kPa) to about 200 psia (1,280 kPag, 1,380 kPa). The abbreviation "psia" denotes pounds per square inch absolute, "kPa" denotes kilopascals, and "kPag" denotes kilopascals gauge (i.e., kilopascals in excess of atmospheric pressure).
The inlet pressure is frequently raised to these levels by compressors, which can introduce lubricating oils in both liquid and vapor form into the gas stream, contaminating the membrane and lower its separation efficiency. To avoid interfering with the membrane operation, the oil content must be reduced to the parts per billion range before the air stream reaches the membrane system. Much of the liquid oil can be removed by relatively coarse units such as coalescing filters, which lower the liquid oil content from an initial 5 to 10 ppm to 1 ppm or less (all such concentrations are on a weight basis). Further reduction of the liquid oil and oil vapor is presently achieved by the use of granular activated carbon, which can lower the oil content to less than 10 ppb.
Many gas mixtures however, particularly compressed air, have a high relative humidity, which is incompatible with activated carbon since the carbon loses its adsorption capability at relative humidities exceeding 50%. Using air as an example, one presently used method of lowering the humidity, after the bulk of the moisture is removed in the compressor unit itself, is to pass the compressed air through a refrigerated dryer and a heater before the air reaches the coalescing filters. The dryer condenses out more water by cooling the air to about 2.degree. C., and the heater returns the air to room temperature so that the resulting air has a relative humidity of about 30%. The refrigerated dryer and heater are cumbersome units that are expensive to run and to maintain, and despite their use, the activated carbon still requires frequent replacement or regeneration. In addition, activated carbon creates dust which can add to the impurities of the air stream and can present an environmental hazard by causing the release of particulate matter into the atmosphere.
For air as well as gas mixtures in general, both cost efficiency and continuity of operation would be improved with a system which did not require lowering the humidity below 50%, which did not require frequent regeneration of the adsorbent, and which can easily be handled without presenting an environmental hazard due to the risk of discharging particulate matter into the atmosphere. These and other goals are met by the present invention.